Clamping sheet grippers are widely used in printing presses to grip a sheet of paper and hold it up against a cylinder. If the paper is pulled out of the gripper even slightly, problems arise with mackling and registration errors occur. Therefore, the gripper typically is required to have a very considerable retaining force, which usually means that the gripper springs must have a very high spring constant. Also, since any play present at the gripper tip would result in registration errors and mackling, it is desirable to minimize the bearing clearances of the gripper elements. The reduced bearing clearance, however, leads to increased friction in the gripper bearings so that some of the spring force operative for gripping is consumed in the bearing itself. The need further arises for the gripper shaft bearings to be very stable in order to reduce deformation associated with the abrupt closure of the grippers. A disadvantage of this is that very high mass forces are produced.
In short, the known gripper systems require very considerable forces for their actuation and only some of such forces can be used for sheet retention. Furthermore, these substantial and abrupt forces may cause unwanted oscillations of the press.
A gripper system of this general kind is shown in DE-PS No. 488 949. A disadvantage of this known system is that the hooked gripper finger is loosely guided between the cylinder wall and guide strip. At high press speeds, only a few milliseconds are available for opening the grippers, and this results in abrupt loads which jar the machine and cause muddy printing. In this gripper, gripper shaft vibrations act both horizontally and perpendicularly to the sheet movement direction, and as a result, cause inaccurate positioning of the delivered sheets.
Another known gripper system is disclosed in DD-PS No. 66 634 wherein a one-piece gripper lever is supported on a gripper shaft and adjustable biasing is provided by two compression springs. A disadvantage of this known system is that the gripper lever loses its statically determined position when the fullest possible compensation for the bearing force is required. At very high press speeds and high biasing forces, centering becomes inadequate, for example, as a result of disturbing vibrations introduced into the press. Other disadvantages are the relatively large inertia radius and the mass of the swinging parts.
DE-OS No. 1 908 181 discloses a gripper system wherein the spindle for the gripper finger pivot is pivotally disposed parallel to the gripper shaft, the spindle of the gripper finger pivot being disposed substantially on the prolongation of a straight line connecting the support surface for the gripper tip to the gripper shaft axis. As is apparent from the geometry shown therein in FIG. 1, the force which the gripper tip applies to the gripper support also has a component in the direction of sheet movement. The sheet may therefore move for this reason and because of possible twisting of the gripper shaft at high biasing, even though there may be some improvement as compared with conventional grippers in which there is an arcuate motion around the gripper shaft axis.
The gripper disclosed in DD-PS No. 67 992 is mounted by means of a clamping member on a pivotable gripper shaft having a stationary axis. A gripper tongue makes a circular movement around such axis in a first movement phase and makes a movement substantially perpendicular to the gripper supporting a second movement phase. This gripper, however, uses a non-positive parallel strip spring arrangement and a gripper tongue which cannot withstand substantial closing forces without buckling. The gripper is therefore completely unsuitable for use with very high closing forces.
Gripper systems of the type disclosed in DE-PS No. 2 030 040 use a perpendicularly closing gripper with a controlled gripper shaft. A disadvantage of this known system is that the non-positive actuation of the gripper shaft relative to the fulcrum of an actuating lever is by means of a guide on a control cam. The additional components associated with the control cam lead to increased mass forces of the system. The components also oscillate with substantial radii of inertia, leading to a reduction in press performance.
The gripper system shown in DE-OS No. 3 130 689 uses a soft gripper support along with a gripper finger which has a flat gripper flight path and which closes perpendicularly in the final movement phase. There is a resilient abutment screw disposed in the gripper finger and operative against the sheet gripper stop. An additional adjusting screw is needed to adjust the resiliently interconnected holders by which the gripper finger is associated with the gripper shaft. A disadvantage of this type of gripper is that the gripper finger must be associated with a soft gripper support and complicated adjustments must be made by means of two adjusting screws to ensure accurate operation. Furthermore, the gripping action becomes uncertain at high press speeds.
The gripper system disclosed in patent application No. P 35 29 639.9-27 uses a resilient clamping gripper which, irrespective of the presence of a soft gripper support, closes perpendicularly in the second movement phase. However, the gripper tip and the retaining surface of the gripper impact strip must be accurately adjusted at the beginning of the final movement phase. Additionally, to prevent abrupt movement during the opening and closing of the gripper, the two springs must be of equal strength.